This invention relates in general to vehicle transmissions and in particular to an improved transmission structure wherein the lower gear ratios are shifted manually and wherein the higher gear ratios can be shifted either manually or automatically as desired.
In most vehicles, a transmission is provided in the drive train between the engine and the driven wheels. As is well known, the transmission includes a case containing an input shaft, an output shaft, and a plurality of meshing gears. Means are provided for connecting selected ones of the meshing gears between the input shaft and the output shaft to provide a desired speed reduction gear ratio therebetween. The meshing gears contained within the transmission case are of varying size so as to provide a plurality of such gear ratios. By appropriate selection of these gear ratios, acceleration and deceleration of the vehicle can be accomplished in a smooth and efficient manner.
Many transmission structures are known in the art for performing this gear ratio selection manually, i.e., in response to some physical exertion by the vehicle driver. In a conventional manual transmission, the driver grasps and moves an upper portion of a pivotable shift lever. In response thereto, a lower portion of the shift lever is moved into selective engagement with one of a plurality of shift rails provided within the transmission. Thus, movement of the shift lever causes movement of the selected one of the shift rails. Movement of the selected shift rail causes certain ones of the meshing gears to be connected between the input shaft and the output shaft so as to provide the desired gear ratio therebetween.
Many transmission structures are also known in the art for performing this gear ratio selection automatically, i.e., without any physical exertion by the vehicle driver. In a conventional automatic transmission, the shift rails are typically replaced by a plurality of hydraulically or pneumatically actuated structures. In response to predetermined operating conditions, these structures cause certain ones of the meshing gears to be connected between the input shaft and the output shaft so as to provide the desired gear ratio therebetween. Although automatic transmissions offer the advantages of increased convenience of use and reduced fatigue for the driver, manual transmissions provide the advantages of decreased cost, increased torque capacity, and better fuel economy. Because of this, the majority of medium and heavy duty truck transmissions in common use today are manual transmissions.
In order to improve the convenience of use of manual transmissions, various structures have been proposed for fully or partially automating the shifting of an otherwise conventional manual transmission. In a fully automated manual transmission, the conventional driver operated shift lever is usually replaced by one or more hydraulically or pneumatically controlled automatic shifting mechanisms. The components of the fully automatic transmission function to shift all of the shift rails within the transmission throughout all of the gear ratios. In a partially automated manual transmission, the conventional driver operated shift lever is usually supplemented with one or more hydraulically or pneumatically controlled automatic shifting mechanisms. The components of the partially automated transmission function to shift only a selected few of the shift rails within the transmission throughout the selected few of the gear ratios which are available from such shift rails. Typically, the lower gear ratios are manually selected by the vehicle driver, while the higher gear ratios are automatically selected. Such an approach is advantageous because the partially automated transmission is lower in cost than a comparable fully automated manual transmission, yet offers automatic shifting in the higher gear ratios where the majority of shifting between gears normally occurs.
As mentioned above, various structures have been proposed for fully or partially automating the shifting of an otherwise conventional manual transmission. However, such known structures have been found to be unduly complicated and expensive. Thus, it would be desirable to provide an improved partially automated transmission which is simpler and less expensive than known structures.